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One of China's most popular social media services, Sina Weibo, filed papers on Friday for an initial public offering in the U.S. with the target of raising US$500 million.

The service, often likened to China's version of Twitter for its 140-character posts, is popular in the world's most populous country and attracted 129 million users per month at the end of 2013, but there are signs of slowing user growth.

In a conference call last month, Sina CEO Charles Chao conceded that competition is tough, with about 20 similar services operating. One of the biggest competitors to Weibo is China-based WeChat, which, like Weibo, claims about 300 million registered users.

Like Twitter, which listed its stock last year, Weibo is pushing mobile. The company already sees about 70 percent of its use from mobile devices, and mobile contributes about 28 percent to the company's advertising revenue, the company said.

Weibo recorded revenue of $188 million last year, of which about 79 percent came from advertising. A further 12 percent came from game services and 6 percent from the sale of premium memberships. The company isn't yet profitable, but its net loss of $38 million last year was much improved from the $102 million net loss in 2012, according to the company.

The U.S. IPO will be handled by Goldman Sachs and Credit Suisse. The filing, which was published by the U.S. Securities and Exchange Commission on Friday, does not list a proposed IPO date or share price. Those are usually determined much closer to the actual IPO.